SOUTH PORTLAND — Local residents determined to prevent the delivery of Canadian tar sands oil through a pipeline to tankers in South Portland warned Tuesday of the potential health hazards, even as a pipeline official again insisted the company has no such plans.

A 236-mile-long pipeline currently transports crude oil from tankers off the coast of South Portland to Montreal. A local group, Protect South Portland, fears that owner Portland Pipe Line Corp. will pursue plans to reverse the pipeline’s flow and deliver “tar sands” — another petroleum product formally called bituminous oil — in the opposite direction, from Alberta, Canada to international markets through the South Portland waterfront.

Tar sands oil is much heavier and thicker than crude oil and must be heated or diluted before it’s pumped through a pipeline.

Opponents of the project fear that could lead to exposure to harmful additives, including benzene, a known carcinogen. Two 70-foot smokestacks would need to be built on the pier next to the city’s Bug Light Park to burn off toxic chemicals before the tar sands oil is loaded onto tankers, while storage tanks also would release pollutants that contribute to smog and lung disease, Protect South Portland members said Thursday at a press event.

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“How many families in South Portland will want to continue living here knowing that they were exposing their loved ones to the threat of lung disease and possibly cancer down the road?” said Cathy Chapman, a local resident and asthma sufferer who plans to move if the project progresses.

Portland Pipe Line President and CEO Larry Wilson previously has said the company has no plans to reverse the pipeline’s flow, though he hasn’t ruled out that move in the future. The company acquired local permits for the project four years ago but let them expire.

On Thursday afternoon, Portland Pipe Line issued a press release announcing the company had formally surrendered its remaining 2009 permit from the Maine Department of Environmental Protection.

“PPLC has been an excellent and honorable corporate citizen in South Portland for many decades,” Wilson said in the release. “I trust the citizens of this great community will accept the surrender of our final 2009 project permit as one more good faith commitment to this city that we do not have a pending or proposed, let alone imminent, ‘tar sands’ project. “

Protect South Portland hopes to thwart any moves to reverse the pipeline’s flow through a new ordinance that will go before local voters on Nov. 5. The proposed Waterfront Protection Ordinance would prevent upgrades to the pier near Bug Light that would be necessary for the project.

“The science is clear,” said Dr. Tony Owens, a Maine Medical Center emergency room physician who lives just over the city line in Cape Elizabeth. “Pollution from tar sands smokestacks is extremely dangerous. It’s simply irresponsible for the oil companies to be denying the serious health effects of tar sands projects.”

The local chapter of the American Lung Association also voiced concerns about the impact on air quality.

Portland Pipe Line’s press release did not address the potential health effects but criticized the ordinance as a measure that “could devastate our working waterfront in South Portland” by costing jobs, economic activity and tax revenue.

The statement came just a day after two local business organizations announced their opposition to the ordinance, calling it “overbroad” and a threat to existing businesses in the region.